Friction braking systems may be inadequate to slow or stop machines in all circumstances. This can occur from travel down an incline, or exist just after the initiation of a directional shift of the machine from a reverse direction to a forward direction, or vice versa. Moreover, when utilized extensively for continuous periods, friction-based braking systems may be susceptible to “brake fade.” Brake fade may be dangerous if the performance of the braking system is less than what is required to stop the machine, particularly when the machine is descending a hill or long incline. Extensive use of brakes also contributes to excessive brake pad wear, which may result in undesirable replacement costs and associated downtime.
Accordingly, many heavy machines include supplementary retarding systems. Conventional propelled machines having mechanical transmissions and drivetrains dissipate portions of undesired power as heat by way of mechanical clutches, torque converters, compression release braking, an exhaust brake, or a hydraulic retarder. The need for power dissipation is enhanced when traveling down inclines inasmuch as the machine may undesirably accelerate.
U.S. Publication 2007/0284170 to Kuras, et al. discloses a method for dissipating power in a propelled machine having a hydromechanical transmission. The method includes converting undesired power to retarding power, and driving the engine with at least a portion of the retarding power prior to dissipating power with any other power-dissipating device. If a measured speed of the machine is greater and not sufficiently close to the speed input from the operator, the level of retarding power required is calculated, and a strategy is executed that engages a hierarchy of devices to providing retarding power. More specifically, in a hydromechanical transmission, retarding power is directed to both the internal combustion engine, and a hydraulic relief valve, and/or a hydraulic powered accessory, and/or an energy storage component.